Failure of Imagination

Whoever funded the 9/11 attacks is of great relevance, but the 9/11 commission concluded, "that's of little or of no significance". It reads as if it were a book written purely of fiction. The conclusion of the official independent 9/11 investigation is that 9/11 was the result of "failure of imagination". I may be losing my cognitive skills possibly, because I don't understand what "failure of imagination" could possibly mean and neither do any of the grieving 9/11 families.